As a technology for forming various films on a substrate in the manufacturing process of a semiconductor device, a chemical vapor deposition (CVD) method or an atomic layer deposition (ALD) method has been used. In these film forming methods, a desired thin film is deposited on a substrate using a chemical reaction by introducing a raw material gas into a processing container that accommodates the substrate.
In the CVD method or the ALD method, a liquid or solid raw material (precursor) is vaporized to produce a raw material gas, and the produced raw material gas is supplied into a processing container. A bubbling method is known as one of the methods for supplying a raw material gas as above. In the bubbling method, the raw material is vaporized by feeding a carrier gas such as an inert gas into a raw material container filled with a volatile raw material. Since the bubbling method is in its nature configured to supply only the portions vaporizable by a self steam pressure to a processing container, it is advantageous in that a particle generation due to non-vaporized components is less as compared to a spray method. In addition, since the bubbling method can be implemented in a device configuration without a part, such as, a spray nozzle having an extremely throttled flow path, there is less concern for the occurrence of clogging in a raw material supply path.
The supply amount of the raw material in the bubbling method (the amount of a vaporized raw material gas in the gas supplied from a raw material container to a processing container. Hereinafter, referred to as a “pickup amount”) qs can be theoretically obtained by the following equation:
                              q          s                =                  k          ×                      (                                                            P                  s                                ⁡                                  (                                      T                    b                                    )                                                                              P                  b                                -                                                      P                    s                                    ⁡                                      (                                          T                      b                                        )                                                                        )                    ×                      q            c                                              [                  Equation          ⁢                                          ⁢          1                ]            
In the equation, k means a number between 0 and 1, which represents the vaporization efficiency of the raw material, Ps(Tb) means the vapor pressure of the raw material at temperature Tb in the raw material container, Pb means the pressure within the raw material container, and qc means the flow rate of a carrier gas.
In the equation above, it is usually difficult to realize the vaporization efficiency k=1 (100%), and k=0.3 to 0.6 (30 to 60%) or so for a low vapor pressure raw material. The pickup amount qs of such raw material varies according to, for example, the height of the liquid level in the raw material container, the flow rate qc of a carrier gas, etc. Specifically, the variation factors of the pickup amount qs include (1) changes in the vaporization efficiency k, (2), changes in the vapor pressure of the raw material due to changes in the remaining amount of the raw material or a raw material alteration in the raw material container, (3) changes in the vapor pressure of the raw material by the disturbance of a heating function, (4) changes in the conductance of a valve on the secondary side along the raw material container to the processing container (for example, changes in the pressure Pb in the raw material container due to the pressure loss), and the like.
Further, from the equation above, it is understood that the pickup amount qs of the raw material also depends on the pressure Pb within the raw material container in a conventional bubbling method where only the portion vaporizable by a self steam pressure is supplied to the processing container. Since the above pressure Pb is almost equal to the pressure of the film forming process in the processing container, it is necessary to determine the process conditions in consideration of the influence on the pick-up amount qs of the raw material. This complicates the construction of process conditions.
With regard to the above problem, Patent Document 1 (Japanese laid-open publication No. 2006-52424) proposes, in the film forming apparatus with a bubbling method, a bubbling system where a mass flow controller (hereinafter, referred to as “MFC”) is installed at a carrier gas supply pipe connected to a raw material container, and a mass flow meter (hereinafter, referred to as “MFM”) is installed at a raw material gas supply pipe which supplies a vaporized raw material gas from a raw material container to a processing container. In this bubbling system, it is possible to monitor the raw material pickup amount qs in the raw material container from the difference between a measured flow rate value by the MFM and a set flow rate value of the MFC. Since the bubbling system proposed in Patent Document 1 allows monitoring of the pickup amount qs of the raw material, it is effective when a factor varying the pick-up amount qs occurs.
In the conventional bubbling method, the interior of a sealed raw material container is saturated under a condition where the processing for a substrate is not being conducted in a processing container (in a standby state). Accordingly, at the time of starting the flow of the vaporized raw material gas in order to process the next substrate, the vaporization efficiency k reaches approximately 1 (100%). However, the vaporization efficiency k converges toward a constant value over time as described above. On the other hand, in a CVD process, for example, in order to secure the uniformity of a film forming processing, it is required to stably feed a predetermined amount of raw material into the processing container over a predetermined time period. Therefore, the conventional film forming process employs a method of discarding the raw material gas by switching to an exhaust bypass line which bypasses the processing container, during the period from a point at which the flowing of the raw material gas is started to a point at which the vaporization efficiency k is stabilized. As such, it takes a considerable amount of time until the vaporization efficiency k is stabilized, thereby causing a reduced throughput or a loss of raw material.